This invention is directed to the control of an ion beam from an ion source by controlling the potential on one or more spoiler electrodes in the plasma chamber.
Ion beams are produced in various ways and are employed for various purposes. One manner of producing an ion beam is by electron-bombardment ionization of a gas. An electron bombardment ion source is described as used in an ion thruster and has been described by H. R. Kaufman in NASA Technical Note D-585 dated January 1960 entitled "An Ion Rocket with an Electron-Bombardment Ion Source." A similar disclosure is found in Kaufman U.S. Pat. No. 3,156,090.
Ion beams are used in ion machining operations. The beams are used to etch material surfaces in order to imprint detailed patterns. These patterns can be used for microfabrication of various devices including metal oxide semiconductor microcircuits. Ion machining is also useful in the manufacture of laser mirrors, acoustic delay lines and the like. Where these patterns extend over a large area or where many patterns are imprinted at one time, a uniform beam flux profile is required over the work area.
Another use of ion beams is in ion implantation. In this operation, impurity atoms are implanted in semiconductor substrates by ion beam injection. Uniform ion beam profiles are required to accommodate a large work area.
In another class of devices, the ion beam is discharged out of a space vehicle, as in electric propulsion of spacecraft. In this type of utilization, it is desirable to have a capability to direct the thrust vector over a few degrees of azimuth. This is useful in spacecraft where the center of gravity may change and it is helpful to maintain the thrust vector in a particular position with respect to the center of gravity. Furthermore, adjusting the direction of the thrust vector is helpful for attitude control operations. Such ion beam thrusters are of the type referred to in the H. R. Kaufman disclosure mentioned above. In such a thruster, a screen electrode and an accelerator electrode are provided at the open end of the plasma chamber to extract and accelerate the beam. Plasma density distribution in the plasma chamber controls the net thrust vector direction.
In such ion beam thrusters, discharge chamber sputtering erosion represents a major failure hazard. The sputtering results primarily from ion bombardment of cathode potential surfaces by doubly charged ions generated near the center of the discharge chamber. The higher density of doubly charged ions near the central region of the discharge chamber is greatly reduced by reducing the central peaking of the plasma density in the beam.
Reduction of central parking is also useful to relax the requirement on beam handling capability (or preveance density P=J/V.sup.3/2) of the beam-extraction system. Although the total ions extracted by the beam-extraction system is typified by the average current density, the system must be designed to be capable of operating at the highest value of current density which occurs locally at the center of the plasma.